Fluid flow when x,y,z are at 0
The x,y,z values in the fluid domain settings define the effect of gravity on the simulation. Values of 0,0,0 mean the fluid is floating in space. A Z value of -9.81 is used to approximate earths gravity.

You have a couple of options for the object used as the starting point of the fluid, setting the type to fluid gives you a single "blob" of fluid with a total volume equal to the size of the mesh while a type of inflow will give you a constant flow of fluid coming in. The initial velocity here defines the direction the fluid first moves, a negative z value will have it fall straight down, while changing the x and y values can get it to flow diagonally into your cup.